1. Field of the Invention
The present invention concerns an information medium which can be written optically along a track the elements of which are shown by prewritten patterns in a surface.
2. Description of the Prior Art
An information medium of this kind typically consists of a disk a light-sensitive surface of which is scanned by an optical beam along a spiral path with a pitch of the order of one micron. It is to be understood that the invention also covers the case of a track the elements of which consist of concentric circles. The light-sensitive surface is scanned by a very small light spot which is used to write data along a turn, in the form of surface irregularities, for example. To assign the writing spot a precise position on the light-sensitive surface, each turn is marked by prewritten patterns which may also consist of surface irregularities. The prewritten patterns generally result from the information medium manufacturing process and because of this their location corresponds to a specific format which defines areas reserved for writing data alternating with prewritten areas which divide up the available space. This particular arrangement is advantageous since the subsequent writing of an information medium in order to store useful information on it has no effect on the service information contained in the prewritten areas.
Service information is that which enables the data along the track to be located and which enables the memory plane to be compartmented using appropriate addresses.
The present invention is more particularly concerned with the prewritten structure which enables the read or write spot to be maintained centered on the axis of the track and which provides the reference timing marks enabling the signals to be sampled with their position along the track taken into account.
In the case of single-spot scanning, it is convenient to provide a prewritten structure comprising offcenter patterns so that a track following error signal may be obtained by comparing samples. A structure of this kind further comprises patterns making it possible to set sampling timing marks in order that the track following error signal has the appropriate sign to maintain the spot correctly centered relative to the track. These auxiliary patterns may be made larger than the read spot so as to be readily distinguished from the other patterns on the information medium, although this solution complicates the provision of the prewritten areas. It is therefore better to distinguish the auxiliary patterns by dividing them according to an exclusive code, rather than by making them larger.
A prewritten structure comprising an arrangment of patterns organised as indicated above is described in patent application Nos. 82 048218, 82 04219, 82 04220 and 82 04221 filed in France on Mar. 12, 1982 in the name THOMSON-CSF. Prewritten structures featuring offcenter patterns are also described in document Nos. EP-A-0 077 641, EP-A-0 077 642, EP-A-0 077 643 and EP-A-0 077 644 published Apr. 27, 1983.
The prewritten structures referred to hereinabove occupy a certain area on the information medium, commensurately reducing the space available for storing usable data. In these known structures the patterns provided for synchronization purposes are situated, ahead of those which are used for track following, which unnecessarily increases the length of the prewritten area.
In order to alleviate this disadvantage, the invention proposes a compact prewritten structure using patterns combining the track following and synchronization functions.